Polyethylenes with a higher density and higher molecular weight are valued in film applications requiring high stiffness, good toughness and high throughput. Such resins are also valued in pipe applications requiring stiffness, toughness and long-term durability, and particularly resistance to environmental stress cracking.
Typical metallocene polymerization catalysts (i.e. those containing a transition metal bound, for example, to at least one cyclopentadienyl, indenyl or fluorenyl group) have recently been used to produce resins having desirable product properties. While these resins have excellent toughness properties, particularly dart impact properties, they, like other metallocene catalyzed polyethylenes, can be difficult to process, for example, on older extrusion equipment. One of the means used to improve the processing of such metallocene catalyzed polyethylenes is to blend them with another polyethylene. While the two polymer blend tends to be more processable, it is expensive and adds a cumbersome blending step to the manufacturing/fabrication process.
Higher molecular weight confers desirable mechanical properties and stable bubble formation onto polyethylene polymers. However, it also inhibits extrusion processing by increasing backpressure in extruders, promotes melt fracture defects in the inflating bubble and potentially, promotes too high a degree of orientation in the finished film. To remedy this, one may form a secondary, minor component of lower molecular weight polymer to reduce extruder backpressure and inhibit melt fracture. Several industrial processes operate on this principle using multiple reactor technology to produce a processable bimodal molecular weight distribution (MWD) high density polyethylene (HDPE) product. HIZEX™, a Mitsui Chemicals HDPE product, is considered the worldwide standard. HIZEX™ is produced in two or more reactors and is costly to produce. In a multiple reactor process, each reactor produces a single component of the final product.
Others in the art have tried to produce two polymers together at the same time in the same reactor using two different catalysts. PCT patent application WO 99/03899 discloses using a typical metallocene catalyst and a conventional Ziegler-Natta catalyst in the same reactor to produce a bimodal MWD HDPE. Using two different types of catalysts, however, result in a polymer whose characteristics cannot be predicted from those of the polymers that each catalyst would produce if utilized separately. This unpredictability occurs, for example, from competition or other influence between the catalyst or catalyst systems used. These polymers however still do not have a preferred balance of processability and strength properties. Thus, there is a desire for a combination of catalysts capable of producing processable polyethylene polymers in preferably a single reactor having desirable combinations of processing, mechanical and optical properties.